Heretofore, several permanent magnet structures have been designed to provide solenoidal magnetic fields. Such structures are basically composed of a permanent magnet shell that forms a solenoidal cavity within which a working magnetic field having a predetermined strength is generated. Depending on the cladding around the shell, these internal working fields can be designed to be uniform or variable along the axial path of the solenoidal cavity.
Examples of such structures can be seen in U.S. Pat. No. 4,692,732 issued Sep. 8, 1987, entitled "Remanence Varying in a Leakage Free Permanent Magnet Field Source," and U.S. Pat. No. 4,701,737 issued Oct. 20, 1987, entitled "Leakage-Free, Linearly Varying Axial Permanent Magnet Field Source," by the present inventor. Both patents disclose a structure having passive ferro-magnets or iron pole pieces placed at either end of a solenoid permanent magnet shell that produces a working field in its solenoidal cavity. The iron pole pieces not only complete the magnetic circuit around the structure, they insure that the surfaces at each end of the solenoidal structure is equipotential. As a result, the surface at each end of the solenoidal structure has no radial change in magnetic potential between the magnetic shell or supply magnet and the axial centerpoint of the solenoidal cavity within which the internal field is confined.
In addition, some embodiments of such structures have cladding magnets of varying thickness and magnetization placed around the outer surface of the permanent magnet shell to insure that the internal field working field does not leak from the internal solenoidal cavity and to enhance the uniformity of the internal field. It is important to note, however, that depending on the shape and magnetization of the cladding magnets and the magnetization of the magnetic shell or supply magnet, the internal field can be made to vary along the axial path of the solenoid, and thus does not necessarily have to be uniform as shown in the two patents cited above. Typically, a hole is bored through the passive ferro-magnet pole pieces to provide access to the internal field whether uniform or gradient.
Although it is desirable for most applications to optimize the uniformity or the gradient of the internal working field, it is not always desirable to use iron or passive ferromagnet pole pieces to complete the magnetic circuit at the ends of such structures to insure that the end surfaces are equipotential as described above. For example, in applications where it is desirable to superimpose magnetic fields from separate sources to form a pattern having a particular distribution, such iron pole pieces would distort or even short out the superimposed field.